A "riot" at Maidstone Prison has been brought under control, the Prison Service has said.
Officers from special prison service riot units across the country were called in to deal with the Maidstone incident in a system known as Operation Tornado.
A prison officers union said they had been told the riot involved up to 180 prisoners, but the Prison Service said that just 40 took part in the "incident".
Prison Officers Association, Vice Chair, Ralph Valerio, told Sky News that negotiators were sent to the scene in Kent to try to contain the disturbance.
He said he had been told that that, at one point, inmates were smashing up Thanet Wing at the prison.
Mr Valerio said he was led to believe that between 160 to 180 foreigners were involved and some staff could have been assaulted.
He said the prison staff were forced to retreat to safety. Thanet Wing, where the riot occurred, is separated from the other wings.
Jackie Hipwell, the landlady of a pub nearby, said the whole road in front of the prison was filled with fire engines and riot vans.
She said it started around 4pm and there were so many vehicles deployed she was unable to use the road.
A Prison Service spokesperson said later: "An incident involving around 40 prisoners at HMP Maidstone has been successfully resolved with no injuries to staff or prisoners.
"An investigation is under way and the perpetrators will be dealt with appropriately by the prison."
Mr Valerio believes staff shortages and the impact this has had on the prisoners is a possible reason behind the rioting.
Maidstone, with an inmate population of about 600, is a category C training prison that predominantly houses sex offenders from the Kent and Sussex areas.
Its website says it aims to "create a therapeutic environment that supports, embraces and empowers change with a primary focus on risk reduction and protection".
The prison also takes in foreign prisoners with more than 18 months to serve and provides forums with the UK Border Agency.
Kent Police confirmed they were aware of the incident, but a spokeswoman said prison services were dealing with the matter.
Meanwhile, a passive demonstration took place at G4S-run Rye Hill Prison, near Rugby, involving 60 inmates. However, this was resolved peacefully and was not linked to the Maidstone riot.